This work uses high precision isotope ratio mass spectrometry to determine the abundance of 13CO2 in respiratory CO2 after the administration of 13C-labeled compounds to neonatal, pediatric, obstetric and lactating patients. In the coming year we will continue instrumental development to reduce the sample volume requirement, so that commonly available 20 ml Vacutainers can be used for collection and storage of analytical samples. The addition of automated LN2 level control to the automated inlet system will permit unattended operation for 18-24 hours, thus reducing the costs of 13C analysis. New tests to assess protein metabolism through the rates at which essential amino acids are oxidized to CO2 will be explored using 1-13C-labeled amino acids. Further kinetic analysis of 13C excretion following the administration of 13C-labeled aminopyrine will be carried out using an advanced computer program for compartmental analysis, adapted from the program SAAM-27 of Dr. Mones Berman.